The present invention relates to a fuel cell, which is an electrochemical cell capable of continuously converting chemical energy generated between fuel and an oxide to electrical energy.
Generally, a fuel cell comprises a stack of several tens to several hundred of unit cells laid one upon another to generate a large quantity of electricity, where each unit cell comprises a pair of counterposed current collector electrodes (separators) and a membrane-electrode assembly (which will be hereinafter referred to as MEA) comprising a polymeric electrolyte membrane and two reactor electrode layers each with a catalyst layer sandwiching the polymeric electrolyte, membrane, and MEA is provided between the separators.
It is preferable that the fuel cell has smaller dimensions particularly in the thickness direction to make the size of the entire cell stack as small as possible. Thus, it is desired to reduce the thickness of the respective constituent parts.
For the separators, materials with an easy current flowability such as carbon, metal, etc. are selected, and carbon is used from the viewpoint of corrosion resistance. The smaller the thickness, the better. Thus, it is desirable that the thickness is not more than about 2 mm, preferably not more than 1 mm. Carbon separators of such a thickness have no elongation property and thus are easily breakable by excessive deformation such as deflection, etc.
Positive or negative reactor electrode layer for use in contact with the separator is made from anticorrosive porous carbon capable of passing hydrogen and oxygen as fuels therethrough. The reactor electrode layer has a thickness as small as about 1 mm or less, preferably about 500 xcexcm or less, more preferably about 300 xcexcm or less and is also porous and thus hardly withstands deformation due to compression, etc.
Polymeric electrolyte membrane is an ion exchange membrane having a thickness as small as about 1 mm or less, preferably about 500 xcexcm or less, more preferably about 200 xcexcm or less, which is even cross-linked and is used in a wet state (gel state). Thus, its strength is small.
Materials for the unit cell constituent parts with such thicknesses are less elongatable and easily breakable by deformation. Thus, rough handling during the cell assembling will give rise to breaking of the constituent parts. Fastening of the fuel cell with a strong force to obtain tight sealing will initiate breaking from the weaker constituent parts.
For the individual unit cells thus formed, it is required to keep the distance between the separators constant and prevent vaporization of water in the polymeric electrolyte membrane, thereby preventing drying of the membrane. TI obtain the necessary sealing to prevent drying, it has been so far proposed to use gaskets (JP-A-7-153480, JP-A-7-226220, JP-A-9-231987, etc.), or use a rubber sheet laminated with a sponge layer as a gasket (JP-A-6-96783, JP-A-7-312223, etc.), or the like.
As to the unit cell fabrication, it is desired that assembling and disassembling of cell constituent members can be made easily, but from the viewpoint of a higher power generation efficiency, assembling of unit cell by curing and fixing the constituent members with an adhesive is a usual means somewhat at the sacrifice of assembling and disassembling workabilities.
However, even in the case of any of the foregoing prior art proposed to obtain tight sealing to prevent drying of the polymeric electrolyte membrane, number of process steps, etc. is considerably increased, resulting in inevitable cost increase, or the resulting fuel cell could not always maintain satisfactory effects throughout the service life. Furthermore, the usual means of assembling the constituent members of unit cell by curing and fixing with an adhesive can attain desired effects only for the initial period, but once the cell members are deteriorated after long service, there will be a difficulty in exchanging the deteriorated members as inconvenience.
An object of the present invention is to provide a fuel cell capable of maintaining a stable power generation efficiency by attaining desired sealing of unit cells, thereby preventing drying of a polymeric electrolyte membrane, with distinguished assembling and disassembling workabilities, easy exchange of deteriorated constituent members, and considerable reduction in production cost.
The present fuel cell comprises a stack of a plurality of unit cells laid one upon another, where the unit cell comprises a pair of counterposed separators, and a membrane-electrode assembly comprising a polymeric electrolyte membrane and two reactor electrode layers each with a catalyst layer sandwiching the polymeric electrolyte membrane, the membrane-electrode assembly being provided between the separators and sandwiched and supported between and by a pair of counterposed resin gasket sheets, and gaskets of inverted V-shape made from cured rubber are integrally formed on outer surfaces of the gasket sheets or inner surfaces of the separators, respectively, and brought into tight contact with the inner surfaces of the separators or the outer surfaces of the gasket sheets, respectively, thereby attaining desired sealing.
In the foregoing structure the membrane-electrode assembly can be supported by a pair of the gasket sheets, and thus the membrane-electrode assembly can be easily and exactly positioned while keeping the specific pressure constant in the unit cell fabrication, thereby considerably improving the working efficiency and handling in the unit cell assembling. Desired sealing can be attained between the separators by the gaskets and maintained stably even if the fuel cell is used as long as its service life. That is, drying due to vaporization of water in the polymeric electrolyte membrane can be prevented and stable power generation efficiency can be obtained.